Smart cards, whether they are contactless and/or comprise contacts, are used inter alia in cashless payment transactions or as access authorization. The smart cards usually comprise a chip module having a chip. The latter is used in the abovementioned case for storing and/or processing the confidential or monetary data. The chip is mounted on a substrate and is usually encapsulated in order to protect the contact-connection and the chip itself against mechanical loading.
Smart cards are often subjected to attacks in order to read out or manipulate the data stored in the chip. The aim of the attacks may also be to determine the chip design for clones. Another conceivable aim of the attack is to use the chip improperly in some other way.
An attack may aim to strip away the chip encapsulation in such a way that the chip is exposed as far as possible without being destroyed and such that it still functions electrically. The unprotected chip can be examined by physical attacks during its operation with regard to its method of functioning and with regard to the stored data. The physical attacks comprise so-called “probing”, in the case of which the signals of the chip are tapped off and evaluated. In the case of so-called “forcing”, the interconnects of the chip are rewired at the microscopic level in order to manipulate the functional sequence of the chip.
On the part of the electrical industry, particularly in the area of pay tv and cashless payment transactions, there is a great interest in preventing attacks on the corresponding chips. Manipulation of the semiconductor chips used for decrypting the pay tv programs transmitted with encryption means that the companies incur high financial losses.
A further area that is particularly affected by smart card manipulations is the credit and cash card sector. The improper manipulation of credit cards or cash cards means that the affected companies or the card holders incur financial damage.
A further area of application in which the smart cards have to be protected against manipulation is the storage of security-relevant or sensitive data, for example in smart cards used as access authorization, electronic passports or patient cards containing data about the medical history.
Conventional encapsulation materials are resistant toward attacks of one specific class of chemical attack, for example to acid attacks or to attacks with bases. With knowledge of the encapsulation material, the encapsulation can be removed in a simple manner by means of wet-chemical attack of an appropriate class of attack that aims to decompose the encapsulation.
Further developments relate to an encapsulation in which a plurality of protective layers are applied to the chip surface. Each of the protective layers affords a limited protection only against one specific class of attack. By analyzing the respective protective layer and using a suitable attack material, encapsulation can be removed layer by layer by selecting a suitable chemical for the wet-chemical attack depending on the layer material.
The use of a completely chemically resistant protective material, for example a glass material, such as borosilicate, or a ceramic, is not possible on account of the high processing temperatures, which would lead to destruction of the chip. What is more, these materials have an unmatched coefficient of thermal expansion and also excessively high modulus of elasticity values, which would lead to the destruction of the chip during operation. Moreover, it is not possible to achieve a sufficient adhesion of these materials on the chip, so that the encapsulation can be lifted off relatively simply from the chip by means of mechanical attacks.